Cathode structure



Sept. 27, 1955 1 R. WALKER 2,719,240

CATHODE STRUCTURE.

Filed March 14, 1946 FIG. I.

VENTOR. LAUREN R. WALKER BY W ATTORNEY United States Patent CATHODESTRUCTURE Laurence R Walker, Cambridge, Mass., assignor, by mesneassignments, to the United States of America as represented by theSecretary of the Navy Application March 14, 1946, Serial No. 654,498

17 Claims. (Cl. 31337) This invention relates to a cathode structure andmore particularly to an improved axial cathode structure for magnetrons.

The use of magnetron tubes in high frequency radio equipment atincreasingly higher voltage levels has made improvement of certainfeatures of the cathode structure mandatory. The method of dissipatingthe excess heat developed at the cathode, because of electronbombardment, by means of radial leads known to the art is inadequate. Inaddition, suificient protection of the leads and elements carrying thehigh voltages from areing or electrical breakdown is very difficult withknown cathode structures.

It is an object of this invention to overcome the abovementioneddifficulties.

It is another object of this invention to provide an axially mountedcathode structure for magnetrons.

Another object is to provide a magnetron cathode structure capable oflarge heat dissipation.

Another object is to provide a magnetron cathode structure which reducesthe danger of high voltage electrical breakdown.

Another object is to provide a magnetron cathode structure with magneticrings to shape and strengthen the magnetic field.

Still another object is to provide a cathode structure for magnetronswith traps in the magnetic rings to minimize migration of the electronemitting material from the cathode to the rings.

A further object is to provide a magnetron cathode structure includingan input jack at one end for receiving a plug carrying high voltage.

A still further object is to provide a cathode structure for magnetronswhich is capable of pre-assembly into a compact unit.

These and other objects will become apparent from the followingspecification when taken in conjunction with the accompanying drawingsin which:

Fig. l is a cross-sectional view of one embodiment of this invention;and

Fig. 2 is a partially cut-away view showing the orienta tion of thecathode structure with respect to the magnetron block.

In Fig. 1 is disclosed a heavy nickel sleeve whose outer surface iscovered with a cathode coating of electron emitting material. Nickelsleeve 10 is brazed at one end to a tapered cylindrical copper piece 12and is also brazed, as shown in Fig. 1, to two toroidal rings 13 and 14which are made of Permendur, a magnetic alloy composed of equal parts ofiron and cobalt wtih a Curie point of 1000 C. While other alloys havingsimilar characteristics may be employed in place of Permendur, thisalloy will be referred to throughout the specification as illustrativeof the characteristics of the alloy to be employed. Permendur rings 13and 14 are undercut to form annular slots 16 and 18 respectively. Amolybdenum sleeve 20 is brazed to the other end of Permendur ring 14,and is zirconium coated to make it a good heat radiator.

Tapered cylinder 12 is brazed as shown in Fig. 1 to copper tubing 22,which is in turn brazed to a circular Kovar piece 24 that has a flangeat its base end. Kovar is an alloy having a coefficient of thermalexpansion such that it permits permanent sealing of meal to glass.Again, other metals having this characteristic are well known. However,the description of the invention will be simplified by referring simplyto Kovar, leaving it to the reader to interpret this term as suggestingother materials having similar characteristics. A Kovar tube 26 issecured to a Kovar cup 28 as shown in Fig. l and a glass seal 30 isformed between cup 28 and flange-like piece 24.

Kovar pieces 24, 26, and 28 with glass seal 30 form an input jack at thebase of the cathode structure to receive a coaxial plug. One lead 34 ofheater coil 32 is securely welded to Kovar cup 28 and is insulated inthe region of tapered cylinder 12 and nickel sleeve 10. The other end ofheater coil 32 fastens to the end of nickel cathode sleeve 10 which isbrazed to sleeve 20.

In Fig. 2 is disclosed an iron block pole piece 35, which contains atapering circular hole through its center thus forming a part of themagnetron block cavity into which the cathode structure is inserted. Theremaining part of the above-mentioned cavity, formed by other parts ofthe tube assembly, is not shown. A Kovar ring 36, which is glassedfirst, fits into the hole in pole piece 35 and is brazed to it along thecircumference of the hole. Kovar ring 36 then is sealed to glassenvelope 38 which in turn is sealed to Kovar ring 40.

The cathode structure including nickel sleeve 10, tapered cylindricalpiece 12, copper tube 22, Permendur rings 13 and 14, sleeve 20, inputjack components 24, 26, 28 and 30, heater lead 34 and heater coil 32, ispreassembled as a unit, which can then be slipped as a whole into themagnetron block and the final seal made by brazing Kovar ring 40 toflange-like Kovar piece 24.

In operation a coaxial plug carrying the heater voltage and cathode highvoltage connects into the input jack formed by Kovar pieces 24, 26 and28. Nickel sleeve 10 heats, causing electrons to be emitted from itscoating. When the magnetron is operating properly, a large number ofthese electrons are returned to the cathode with sufficient force togenerate considerable amounts of heat. This heat is dissipated throughnickel sleeve 10, tapered cylindrical copper-piece 12 and copper tube 22by conduction and by radiation from zirconium-coated molybdenum sleeve20. In addition to increasing the total heat dissipation, sleeve 20serves to equalize the amount of heat dissipation at each end of thecathode. The amount of heat conduction from the cathode is increased bymaking the walls of nickel sleeve 10 and copper tube 22 thick and makinggood contact between nickel sleeve 10, copper-piece 12 and copper tube22. The amount of heat conduction can be controlled by controlling thedimensions of copper tube 22.

Copper tube 22 is of sufficient size to insure mechanical rigidity ifthe brazed joints are properly made. Good electrical breakdownproperties are achieved by making the whole structure consist ofinter-locking cylinders, which present no sharp points. The radii of thecylinders are as large as consistent with the clearances required insidethe tube, thus making the field gradients at the cathode structure aslow as possible.

The Permendur rings 13 and 14 serve to prevent leakage of electronsalong the axis of the tube. From this function they derive the name ofhats. By using Permendur with a Curie point of 1000 C., the hats retaintheir magnetic properties when the magnetron is operating. Permendurhats 13 and 14 are shaped to produce a nearly uniform magnetic fieldover a large fraction of the center of .the inter-electrode :gap and afocusing field over the remainder. Because they are located between thenorth and south poles of the magnet, Permendur hats 13 and 14 alsocontribute substantially to the strength of the magnetic field in theanode-cathode gap and thereby allow a decrease in total magnet weight.

Annular slots 16 and 18 increase the path length of migration of cathodecoating material from the cathode to Permendur hats 13 and 14respectively, and thereby cut down possible emission from the hats. Suchemission is undesirable from the standpoint of loss of efliciencyandother possible effects deleterious to good operation of themagnetron.

The method of dissipating heat by inter-locking cylinders of large areasis readily seen to be far superior to that of using radial leads. Byintroducing the high voltage onto the cathode through thehigh voltagecoaxial jack at one end of the cathode structure only, the problem ofclearance and shielding of the high voltage lead is greatly simplified.

The novel features .of this invention include a cathode structure formagnetrons which (a) can be completely pre-assembled before inserting inthe magnetron, with no welding operations required in mounting thecathode in the tube; (b) gives large, and controlled heat dissipation;(s) can be made rigid mechanically; (d) eliminates all sharp pointswhich might contribute to electrical breakdown; (2) includes magneticalloy rings to shape and strengthen the magnetic field; and (f)terminates at one end in a high voltage coaxial jack.

This invention is only to be limited by the appended claims.

What is claimed is:

1. A unitary cathode structure comprising a cylindrical cathode providedwith a coating of electron emitting material, supporting means securedto one end of said cathode in axial alignment therewith and adapted tosupport said cathode, said cathode being supported solely by saidsupporting means, and heat dissipating means secured to said cathode atthe end of said cathode opposite said supporting means, said heatdissipating means being supported solely by said cathode.

2. An improved axial cathode structure comprising a cathode with acoating of electron emitting material, and means shaping andstrengthening a magnetic field, said means being disposed on either endof said cathode in axial alignment therewith, said means and saidcathode being formed as interlocking cylinders rigidly fastenedtogether.

3. A cathode structure comprising, an elongated cathode having an axialopening therein, and means constructed of magnetic material for shapingand strengthening a magnetic field disposed on the end of said cathodein axial alignment therewith, said means being shaped to prevent leakageof electrons along said cathode structure.

4. A cathode structure of the type described in claim 3 wherein saidmeans is provided with an annular slot concentric with said cathode.

5. An improved axial cathode structure comprising a cathode formed witha coating of electron emitting material, supporting means secured tosaid cathode structure in axial alignment therewith and heat dissipatingmeans secured to said cathode at the end of said cathode opposite saidsupporting means, said supporting means, said cathode and said heatdissipating means being formed as interlocking cylinders, rigidlyfastened together.

6. An improved axial cathode structure comprising a cathode with acoating of electron emitting material, means shaping and strengthening amagnetic field, said means being disposed on either end of said cathodein axial alignment therewith, heat dissipating means positioned as anaxial extension of one of said field strengthening means, saidaforementioned elements being formed as interlocking cylinders rigidlyfastened together.

7. A unitary cathode structure for a magnetron comprising an elongatedcylindrical cathode formed with a coating of electron emitting material,a supporting means secured to said cathode structure in axial alignmenttherewith, said supporting means being formed with a portion thereofremote from said cathode adapted to be fastened to a magnetron anode,said supporting means further including means electrically insulatingsaid last-mentioned portion from said cathode, and an elongatedcylindrical heat dissipating means secured to said cathode in axialalignment therewith at the end of said cathode opposite said supportingmeans.

8. An improved axial cathode structure for magnetrons capable ofpreassembly into a single unit comprising, a cathode formed with acoating of electron emitting material, supporting means secured to saidcathode at one end thereof and in axial alignment therewith, the end ofsaid supporting means remote from said cathode being formed as the outershell of a coaxial connector, magnetic field shaping and strengtheningmeans mounted on said cathode intermediate said coating of said emittingmaterial and said supporting means, said field strengthening means beingformed to prevent leakage of electrons along the axis of said cathodestructure, said field strengthening means being securely fastened tosaid cathode whereby good electrical breakdown properties and rigidityare obtained, and means mechanically and insulatively securing saidsupporting means into a magnetron block.

9. An improved axial cathode structure comprising a cathode formed witha coating of electron emitting material, means shaping and strengtheninga magnetic field, said means being disposed on either end of saidcathode in axial alignment therewith, said field strengthening meansbeing shaped to prevent leakage of electrons along said cathodestructure, heat dissipating means positioned as an axial extension ofone of said field strengthening means, said aforementioned elementsbeing formed as interlocking cylinders rigidly fastened together wherebysaid structure is mechanically rigid and has good electrical breakdownproperties.

10'. A cathode structure as in claim 9, said structure furthercomprising a coaxial connector disposed in axial alignment with saidcathode, means electrically and mechanically connecting said structureto the outer shell of said coaxial connector thereby to form apre-assembled unit, and means securing said pre-assembled unit into amagnetron block, said means including means for insulating saidpre-assembled unit from said block.

11. An improved axial cathode structure for magnetrons, capable ofpre-assembly into a single unit, comprising a cathode with a coating ofelectron emitting material, a heater coil disposed internally of saidcathode, connector means having one terminal thereof connected to saidcathode and first and second terminals thereof connected to oppositeends of said heater coil whereby said cathode may be connected to asource of high voltage and said heater coil may be connected to a sourceof low voltage, heat dissipating means disposed adjacent said cathode inaxial alignment therewith, means shaping and strengthening a magneticfield, said means being disposed as axial extensions of said cathode,means intermediate said cathode and said field strengthening meanspreventing leakage of electrons along the axis of said cathode, saidaforementioned elements being rigidly fastened together, and meanssecuring said pre-assembled unit into a magnetron block, said lastmentioned means including means for electrically insulating saidpre-assembled unit from said block.

12. An improved axial cathode structure for magnetrons capable ofpro-assembly into a single unit comprising a cathode formed with acoating of electron emitting material, supporting means secured to saidcathode at one end thereof and in axial alignment therewith, the end ofsaid supporting means remote from said cathode being formed as the outershell of a coaxial connector, heat dissipating means secured to saidcathode in axial alignment therewith and at the end, of said cathodeopposite said supporting means, magnetic field shapingand strengtheningmeans mounted on said cathode intermediate said coating of emittingmaterial and said supporting means and said coating and said heatdissipating means, respectively, said field strengthening means beingformed to prevent leakage of electrons along the axis of said cathodestructure, said aforementioned elements being formed as interlockingcylinders securely fastened together whereby good electrical breakdownproperties and rigidity are obtained, a heater coil mounted internallyof said cathode, and having one end thereof electrically connected tosaid cathode, an electrically conductive member insulatively mountedwithin said supporting means and having one end thereof connected to asecond end of said heater coil, the end of said conductive member remotefrom said heater coil being formed as the inner member of a coaxialconnector, said above-recited elements comprising a unit capable ofpre-assembly, and means mechanically and insulatively securing saidsupporting means into a magnetron block.

13. A cathode structure as in claim 12 wherein said last mentioned meansis in the form of a cylinder surrounding said supporting means andsealed about its peripheries to said magnetron block and said supportingmeans respectively thereby providing a vacuum seal of the opening in theblock within which the cathode structure is located.

14. A unitary cathode structure adapted to be secured in an openingformed in the anode block of a magnetron, said cathode structurecomprising an elongated cylindrical cathode having a coating of electronemissive material, a supporting means having the form of two cylindricalconcentric members electrically and mechanically joined at one endthereof, the inner member of said two concentric members being supportedsolely by said joined end, said cathode being secured to the unjoinedend of said inner cylindrical member in axial alignment therewith, theunjoined end of said outer cylindrical member being formed forattachment to said anode block at said opening therein, said cathodestructure being adapted to form a gas tight closure for said opening insaid anode block, said outer cylindrical member including meanselectrically insulating said unjoined end of said outer cylindricalmember from said joined ends of said inner and outer cylindricalmembers, said cathode and said inner member of said supporting meanshaving communicating axial opening therein, said joined ends of saidinner and outer cylindrical members of said supporting structure beingformed as the outer member of a coaxial jack, said cathode structurefurther comprising a heater coil disposed within said cathode, meansforming the inner member of a coaxial jack insulatedly supported withinsaid inner member of said supporting means, and means electricallyconnecting said means forming the inner member of said coaxial jack andone terminal of said heater coil, and a second terminal of said heatercoil being connected to said cathode.

15. An improved axial cathode structure for magnetrons, capable ofpre-assembly into a single unit, comprising a cylindrical cathodeincluding a coating of electron emitting material, an input jack, aheater coil, one lead of said heater coil connecting to one terminal ofsaid input jack, said input jack being located at one end only of saidcathode structure, a second lead of said heater coil connecting to saidcylindrical cathode internally, said leads being ultimately connected toa source of heater voltage, said cylindrical cathode being brazed to atapered cylindrical copper plate, said tapered cylindrical copper platebeing brazed to a cylindrical copper tube, said cylindrical copper tubebeing brazed to a second terminal of said input jack, said input jackbeing adapted to receive a coaxial plug, said cylindrical copper tubebeing of sufficient strength to support said cathode structure rigidly,means disposed at either end of said cylindrical cathode shaping andstrengthening a magnetic field, means associated with said cylindricalcathode preventing leakage of electrons along the axis of said cathodestructure, and means securing said pre-assembled unit into a magnetronblock.

16. An improved axial cathode structure for magnetrons, capable ofpre-assembly into a single unit, comprising a cylindrical cathodeincluding a coating of electron emitting material on its outer surface,a heater coil, an input jack, a lead connecting heater coil to a centerterminal of said input jack, said input jack being located at one endonly of said cathode structure, a second lead of said heater coilconnecting to said cylindrical cathode internally, said leads beingultimately connected to a source of heater voltage, said cylindricalcathode being brazed to a tapered cylindrical copper plate, said taperedcylindrical copper plate being brazed to a cylindrical copper tube, saidcylindrical copper tube being brazed to a second terminal of said inputjack, said second terminal being connected ultimately to a source ofhigh voltage and to one side of a source of heater voltage, said inputjack being adapted to receive a coaxial plug, said cylindrical coppertube being of suflicient strength to support said cathode structurerigidly, said cylindrical cathode, tapered cylindrical copper plate, andcylindrical copper tube being of sufficient area and being brazedtogether sufficiently well to provide an adequate path for conduction oflarge amounts of heat, said cylinders overlapping to present smooth,even surfaces with good electrical breakdown properties, a pair of hats,toroidal in shape, brazed to said cylindrical cathode, said hats beingof magnetic material to help shape and strengthen a magnetic field whichlies parallel to said cylindrical cathode, said hats being oriented toprevent electrons from escaping along the axis of said cathodestructure, said hats being undercut to lengthen the path of migration ofsaid electron emitting material along said external surface of saidcylindrical cathode, and means for securing said pre-assembled unit intoa magnetron block.

17. An improved axial cathode structure for magnetrons, capable ofpre-assembly into a single unit, comprising a cylindrical cathodeincluding a coating of electron emitting material, a heater coil, aninput jack, a lead to said heater coil connected to a center terminal ofsaid input jack, said input jack being located at one end only of saidcathode structure, a second lead of said heater coil connecting to saidcylindrical cathode internally, said leads being ultimately connected toa source of heater voltage, said cylindrical cathode being brazed to atapered cylindrical copper plate, said tapered cylindrical copper platebeing brazed to a cylindrical copper tube, said cylindrical copper tubebeing brazed to a second terminal of said input jack, said input jackbeing adapted to receive a coaxial plug, said cylindrical copper tubebeing of sutficient strength to support said cathode structure rigidly,said cylindrical cathode, tapered cylindrical copper plate, andcylindrical copper tube being of sufficient area and connected togethersufiiciently well to provide an adequate path for conduction of largeamounts of heat, said cylinders overlapping to present smooth, evensurfaces with good electrical breakdown properties, a pair of hats,toroidal in shape, brazed to said cylindrical cathode, said hats beingof magnetic material to shape and strengthen a magnetic field which liesparallel to said cylindrical cathode, said hats being oriented toprevent electrons from escaping along the axis of said cathodestructure, said hats being undercut to lengthen the path of migration ofsaid electron emitting material along said external surface of saidcathode, a zirconium coated molybdenum heat radiator, cylindrical inshape, being brazed to one of said pair of hats, a cup being brazed to amagnetron block, a glass envelope, a second cup, said first cup beingsealed to said glass envelope which is sealed to said second cup, saidsecond cup being brazed to said input jack for final seal of saidpre'assembled unit to said magnetron block.

(References on following page) References Cited in the file of thispatent UNITED STATES PATENTS Holhnann Mar. 28, 1939 Bondley June 17,1947 Nelson July 22, 1947 8 McArthur Aug. 10, 1948 Spencer Oct. 26, 1948Derby Mar. 8, 1949 Wax Apr. 12, 1949 Kather Aug. 9, 1949

